1. Field of the Invention
The invention consists principally of an Instantaneous Frequency Measurement Receiver (hereinafter called an IFM), and an Instantaneous Fourier Transform Receiver (hereinafter called an IFT) operating in a parallel and complementary manner controlled by other signal processing and control elements which enable the IFM and the IFT to function in a synergistic manner to produce a rapid identification of the frequencies of incoming radar signals, and the like.
2. Description of Related Art
U.S. Pat. No. 4,075,630 issued Feb. 21, 1978 to Gerald N. Shapiro, et. al. for a "Signal Processor".
This patent pertains to apparatus for detecting pulse doppler signals, then removing them by frequency filtering. It uses a Fast Fourier Transform Processor which is a different apparatus from an Instantaneous Fourier Transform Receiver. A discussion of Fourier transforms is presented to distinguish a Fast Fourier Transform Processor from an Instantaneous Fourier Transform Receiver.
See the book "Theory and Application of Digital Signal Processing" by L. Rabiner & Gold, published by Prentice Hall in 1975 for a thorough discussion of the relation between Finite Fourier Transforms, Fast Fourier Transforms and Fourier series as applied to periodic function as contrasted to Fourier Integrals which do not pertain to periodic functions.
A Finite Time to Frequency Fourier transform is a mathematical transformation transforming a periodic function of time into a function of frequency. A finite number of, preferably uniformly, spaced apart samples of amplitude of the time function are taken over one period of the time function. Finite Fourier Transforms are made, using those samples, to obtain the coefficients of the terms of an infinite sine, cosine or e.sup.j .OMEGA..sup.t series. One term of the series is the dc or zero frequency component, and the remaining terms of the series have angular frequencies corresponding to the fundamental angular frequency, .OMEGA., of the time-periodic function which was sampled and an infinite number of harmonics thereof. Some of the coefficients may be zero. When the Fourier Series is properly factored, the number of steps for obtaining the coefficients is reduced, and the transforms are called "Fast Fourier Transforms".
When the time function to be transformed is not periodic, a Fourier Integral is used to obtain a measurement of the frequency function that corresponds to the time function.
An Instantaneous Fourier Transform (IFT) Receiver performs a Fourier Integral transformation of a pulse or CW into its component frequencies, and such transformation is extremely rapid compared, for example, to the transformation of a signal by a Fast Fourier Transformation Apparatus. The IFT may be an acousto-optic device, but it is intended to include other apparatus which will or does operate with the speed of such device to produce simultaneous and instantaneous frequency information. A Bragg Cell used as an IFT displays all of the component frequencies in, for example, one or two nanoseconds after receipt of a radar pulse.
An Instantaneous Fourier Transform Receiver, is a predetection apparatus. The incoming signal is first separated simultaneously into its frequency components, and those components are simultaneously detected or sensed. A good discussion of such transformations may be found in the book "Acousto-Optic Signal Processing" edited by Norman J. Berg and John N. Lee, published by Marcel Dekker, Inc., 1983. A discussion of Acousto-optical Fourier transformation commences on page 19.
The apparatus of the Shapiro, et. al. patent, however, is a Fast Fourier Transform apparatus that is designed to detect pulse doppler signals and to filter them out. It is a post-detection device. The signals are first detected, then the signal is resolved by an iterative process in a computer into its frequency components. Such apparatus must be preceded by high-speed analog/digital conversion pulse sample and hold circuitry. It may not be assumed that the analog/digital means is already incorporated in the FFT box. Further, such apparatus are subject to aliasing effects which do not appear in an IFT.
U.S. Pat. No. 4,208,632 issued June 17, 1980 to Edward J. Sheldon, et. al. for a "Radar Receiver".
The Sheldon, et. al patent pertains to a radar receiver which includes a plurality of intermediate frequency and video frequency amplifiers with each of the amplifiers arranged to process received signals within a different range of amplitudes so that the overall dynamic range of such receiver may be equivalent to the dynamic range of a conventional radar receiver with automatic gain control.
U.S. Pat. No. 4,336,541 issued June 22, 1982 to James B. Y. Tsui, et. al. for a "Simultaneous Signal Detector for an Instantaneous Frequency Measurement Receiver".
This patent pertains to a Simultaneous Signal Detector (SSD) apparatus for use with an Instantaneous Frequency Measurement (IFM) Receiver having an alarm signal indicating that the output frequency signal of the IFM is ambiguous because the IFM has latched onto two signals. So, too, in the apparatus of this invention, it is desirable to determine when the DIGITAL FREQUENCY output of the IFM is ambiguous, and to tag ambiguous frequency readings. An SSD is used for that purpose, but the preferred apparatus is different than that shown in the patent.